Neighbor built fence on my property

What is the name of your state? Florida

I built a fence a few feet on my side of the property line so as to avoid some trees. My neighbor already had a small fence that ran parallel to my fence. Sometime after the hurricanes, he removed the parallel portion and extended the other side of his fence to connect to mine. Basically, he has fenced some of my property into his backyard.
I don't actually mind that he is using this portion of my property. However, I am concerned that after some length of time, he could claim that property as his own. I am also concerned that this could be an issue if he or I try to sell our houses. Could I have him sign a simple agreement recognizing that is my property and that I could ask for him to move his fence? Would that sufficiently address my above concerns?

Send him a certified (RRR) letter demanding that he either remove his fence from your property or sign an agreement giving up any ownership rights to the kidnapped portion of your land.

If he refuses, sue his ass.